1. Field of the Invention
This invention relates to a grinding method and a grinding machine to grind a workpiece which has end faces at both sides of a cylindrical part.
2. Discussion of the Background
It is well-known that a workpiece W, e.g., a crankshaft, has a cylindrical part 51, a pair of end faces 52 at both sides of the cylindrical part 51 and R-parts 53 connecting the cylindrical part 51 with the end faces 52. FIG. 6(a) shows a conventional grinding method of the workpiece W whose rotational axis is parallel to the rotational axis of a grinding wheel 50. A grinding stone of the grinding wheel 50 is formed into a shape corresponding to the finished shape of the cylindrical part 51, end faces 52 and R-parts 53 of the workpiece W by truing. According to the method, a single plunge grinding step completes grinding the cylindrical part 51, end faces 52 and R-parts 53 so as to reduce a grinding time. However, a grinding amount per unit area is larger between the sides (edges) of the grinding stone of the grinding wheel 50 and the R-parts than at a circumference of the grinding stone, so that the end faces 52 of the workpiece W are heated up and tend to obtain a grinding burn. As to the formed grinding wheel 50, because of the large grinding amount around the edges, the edges partially wear off as shown in FIG. 6(b). Although the grinding stone of the grinding wheel 50 is modified by the truing, the width of the grinding stone is set equal to the finishing width between the end faces 52 so that the sides of the grinding stone are not trued. The reason is that truing the sides make the grinding stone of the grinding wheel 50 thinner, so that one plunge grinding process cannot create the finishing width between the end faces 52. Therefore, the grinding stone of the grinding wheel 50 is modified by truing on the circumference and the R-parts toward the two-dot chain line shown in FIG. 6(b), which eliminates a large amount of the grind stone. This results in poor productivity with respect to the number of workpieces W to be ground per grinding wheel 50.
Japanese patent application publication No. 2005-324313 discloses another grinding method with a grinding wheel whose grinding stone is thinner than the width between the end faces. The method has a first grinding step in which the grinding wheel is fed to one of the end faces while moving obliquely toward the other end face so as to grind conically, and a second grinding step in which the grinding wheel traverses to the one end face parallel to the rotational axis of the workpiece and then retracts vertically so as to eliminate the cone and finish the end face. According to this method, however, at least two plunge grinding steps are required, so that the grinding time becomes long. Additionally, because the grinding width is large in the beginning of the grinding, as is the grinding volume, the workpiece is heated up and expanded, resulting in poor precision.
Further prior art is disclosed in Japanese patent application publication No. 55-137865. This discloses a so-called an angular grinding machine whose grinding wheel rotates about a rotational axis inclining to the rotational axis of the workpiece. The grinding wheel of the angular grinding machine has a cylindrical grinding portion and a face grinding portion, so that the cylindrical part and end face of the workpiece are ground by alternately feeding the workpiece in the direction of its rotational axis and the grinding wheel in the direction of the inclination to the rotational axis of the grinding wheel. The angular grinding machine, however, is able to grind only one of the end faces unless the workpiece is reversed, whereby the grinding time is increased. Or, depending on the width and depth of the pair of the end faces, the grinding wheel is not able to be fed into the intermediate part of the end faces.